This invention relates to the design of optical coatings for controlling the manner in which light of particular wavelengths is transmitted by or reflected from an optical surface.
The phenomenon of optical interference, which causes modifications in the transmitted and reflected intensities of light, occurs when two or more beams of light are superposed. The principle of superposition states that the resultant amplitude is the sum of the amplitudes of the individual beams. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves. The light waves are reflected at opposite surfaces of the thin film of soap solution or oil.
One important practical application for interference effects in thin films involves the production of coated optical surfaces. If a film of a transparent substance is deposited on glass, for example, with a refractive index which is properly specified relative to the refractive index of the glass and with a thickness which is one-fourth of a particular wavelength of light in the film, the reflection of that wavelength of light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a nonreflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the antireflective performance of such films by using a composite film having two or more superimposed layers. In theory, it is possible with this approach to design a wide range of multiple-layer interference coatings for obtaining a great variety of transmission and reflection spectrums. This has led to the development of a large number of new optical devices making use of complex spectral filter structures. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band-rejection filters are some of the examples of useful devices employing thin-film interference coatings.
Frequently two different materials are used in fabricating such a composite film, one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited to specified thicknesses to obtain the desired optical characteristics for the film. The deposition process is typically controlled by monitoring the thickness of each layer as it is deposited and terminating the deposition when the layer reaches the correct thickness. Some advanced applications of optical technology, however, require antireflective films which adhere more closely to theoretically specified refractive index profiles, to exhibit even lower levels of reflection than have previously been attainable in the art. The use of layers having intermediate values of refractive index, which requires the coevaporation of two materials, or of a gradient index coating, in which the index of refraction within the coating is made to vary continuously as a function of depth in the layer, further increases the degrees of freedom available in the design of such films. When the coating design calls for a layer of intermediate index or a gradient-index layer, the thickness monitoring technique of depositing optical coatings may not be sufficient to ensure the accuracy of the deposited layer.